I've walked through that a while ago! It's underneath Amsterdam Central on its western side. AnSionnachRua go check that shit out. The murals are as Dutch as can be, in blue-white Delftware pottery style. Delftware is so quintessentially Dutch that when you fly KLM business class, you get a Delftware house. (Filled with jenever, of course.)

Benthem/Crouwel are one of the better Dutch architects. Notice how they took the vertical break in the balustrade above the tunnel and used it for the shape of the tunnel? I love details like that. Another cool thing is that the tunnel on this side has a mural based on a 400 year old painting, but on the other side it is neat and modern tiles. When walking the tunnel, the mural transitions from the old to the new, representing the change Amsterdam has gone through. (Or at least that's the idea behind it.)